(1) Field of the Invention
The present invention relates to a device for magnetic resonance (MR) imaging and spectroscopy of fluid-based objects, generally. More particularly, the present invention relates to a new slow “magic angle” spinning (slow-MAS) probe useful for Magnetic Resonance (MR) imaging and spectroscopy of biological objects, including live animals.
(2). Description of the Technical Art
In vitro and in vivo 1H NMR spectroscopy are widely used for investigating biochemical processes in cells, tissues, animals, and humans (see, for example, I. R. Young, ed., methods in Biomedical magnetic resonance imaging and spectroscopy). However, the 1H spectrum obtained from a static sample often suffers from poor resolution due to various line broadening mechanisms inherent within a biological system. This hampers a quantitative, and sometimes even qualitative, analysis of the spectra. And, it is found that when intact objects such as cells, cell systems, tissues, organs, and living specimens (e.g., “biological objects”) are placed in an external magnetic field, variations in the isotropic bulk magnetic susceptibility near the boundaries of inter- and intra-cellular structures induce local magnetic field gradients in the objects. While these susceptibility gradients can sometimes provide useful medical information, they also broaden the NMR lines. This is especially true for 1H NMR spectroscopy where resulting spectra often contain severely overlapping spectral lines that can seriously hamper both the qualitative and quantitative spectral analysis. Increasing the T2 weighting factors can sometimes enhance the spectral resolution, but can also result in serious signal losses and spectral distortions.
It is well known in the art that susceptibility broadening can be averaged to zero by the technique known as Magic-Angle Spinning (MAS), whereby a sample or specimen is rotated about an axis positioned at an angle of 54°44′ relative to the external magnetic field B0. In a standard MAS experiment, a single 90° radio frequency (RF) pulse is used to observe the signal. The spinning frequency is typically chosen equal to or greater than the spectral width to avoid spectral spinning sidebands (SSBs) that can surround the various resonance peaks. Of particular concern are SSBs associated with the residual water signal. For example, if a lower spinning frequency is used, water SSBs can overlap with the metabolite lines rendering the interpretation of the spectra difficult. Spinning rates varying from several kHz to more than 10 kHz have been used to obtain high-resolution 1H MAS metabolite spectra in cells and excised tissues. However, techniques are required that separate or eliminate the SSBs from these high-resolution metabolite spectra.
A serious problem associated with conventional fast-MAS spectroscopy of fluid-filled objects (including live specimens) is the introduction of large centrifugal forces (Fc) that can be induced in a sample from the high spinning rates. For example, spinning can destroy the tissue structure and even individual cells. The centrifugal force Fc is given by Fc=mω2r, where m is the mass, ω=2πF, F is the spinning frequency, and r is the distance from the rotational axis to the point of interest. As an example, when F=2 kHz and r=1 cm, Fc can equal 1.6×105 times the gravitational force G. Thus, standard MAS spectroscopic methods are not viable for the study of intact and/or larger biological objects, samples, or specimens and more particularly in vivo studies. Rotating a specimen under slow-, or ultra-slow, spinning speeds will not affect the fluid object. Gravitational forces are low in such experiments, even in larger objects such as animals. For example, a small mouse spun at a frequency of 1 Hz experiences a maximum gravitational force effect at its perimeter of only 0.04 G (at a maximal distance of about 1 cm from the rotational axis). And, centrifuge experiments conducted on dogs and rats [see, for example, Oyama et al., Response and Adaptation of Beagle Dogs to Hypergravity, Life sciences and Space Research XIII: Proc. of the 17th Plenary Meeting, Sao Paulo, Brazil 1974, Akademie-Verlag, Berlin, 1975. p. 11–17 and Wunder et al., Knee-Ligament Loading Properties as Influenced by Gravity: I-Junction with Bone of 3-G Rodents: Aviation, Space, and Environm. Med. 1982; 53: 1098–1111) at the maximum allowable centrifugal force (a value of 2.5–3 G) suggest that a mouse may be spun at speeds of up to 8.6 Hz.
Commercially available magic angle spinning (MAS) probes are largely aimed at achieving sample spinning rates on the order of several hundred Hz or greater because the primary application of the MAS probes is almost entirely applied in the field of solid state NMR. Thus, the current state-of-the-art teaches, and is directed to, increasingly higher spinning rates and increasingly higher magnetic field strengths in order to eliminate the spinning sidebands associated with various internal spin interactions. And, instruments with sample spin rates as high as 26 kHz, for example, are commercially available (Doty Scientific Inc.).
The probe of the present invention contrasts with the current state-of-the-art in that the selection of spin rates is at substantially lower spin frequencies so as to be operable for MR Imaging and Spectroscopy on fluid-objects, and more preferably on biological objects.
In a recent attempt to test the applicability and viability of commercial systems, we made modifications to an existing Varian-Chemagnetics probe; spinning speeds as slow as 1 Hz were achieved. However, a slow, stable spin rate was not achieved in the modified commercial NMR probe over a sufficiently long time interval to be viable. Further still, the commercial probe routinely overshot the upper frequency boundary limit of 100 Hz for safe spinning upon initial startup, which proved unacceptable for research involving live objects, including live animals.
Recently we have been able to separate SSB's from the metabolites spectrum of interest in fluid objects by combining slow-MAS with special radio frequency (R.F.) pulse sequences using two methods originally developed for solid state NMR (Antzutkin et al. in J. Magn. Reson. 1995, A115: 7–19; and Hu J Z, Wang W, Liu F, Solum M S, Alderman D W, Pugmire R J, Grant D M. Magic-angle-turning experiments for measuring chemical-shift-tensor principal values in powdered solids. J. Magn. Reson. 1995, A113: 210–222). These include: 1) Phase Adjusted Spinning Sidebands (PASS) and 2) Phase-corrected Magic Angle Turning (PHORMAT). For example, we have modified and successfully applied two-dimensional sideband separation in magic-angle-spinning NMR on biological samples (R. A. Wind, J. Z. Hu, and D. N. Rommereim, “High Resolution 1H NMR Spectroscopy in Organs and Tissues Using Slow Magic Angle Spinning”, Magn. Reson. Med. 46, 213–218 (2001), J. Z. Hu, D. N. Rommereim, and R. A. Wind, “High Resolution 1H NMR Spectroscopy in Rat Liver Using Magic Angle Turning at a 1 Hz Spinning Rate,” Magn. Reson. Med. 47, 829–836 (2002)) hereby incorporated by reference.
It was found with PASS that spinning speeds as low as 40 Hz could be used, which makes the techniques amenable for small fluid objects such as cell agglomerates, excised tissues and organs. Similarly, with PHORMAT, the spinning speed could be reduced to 1 Hz, albeit with less sensitivity and a longer measuring time than with PASS. Still, PHORMAT is the only technique available to date for studying larger objects with slow MAS, including live animals. PHORMAT was recently demonstrated in a live mouse (R. A. Wind, J. Z. Hu, and D. N. Rommereim, “High resolution 1H NMR spectroscopy in a live mouse subjected to 1.5 Hz magic angle spinning,” Magn. Reson. Med. (2003)). Previous patent applications regarding the slow-MAS methodology have been submitted (see US2002-0135365A1 and US2002-0125887A1), hereby incorporated in their entirety herein by this reference. Although these applications comprehensively describe the general procedures of the slow-MAS methodology, nowhere do they describe a slow-MAS probe suitable for applying the methodology to a fluid biological object.
Accordingly, there remains a need for a “magic-angle” spinning probe useful for high resolution magnetic resonance imaging and spectroscopy of fluid objects. More specifically, there remains a need for a slow Magic Angle Spinning probe that allows for the mounting of biological objects, including live intact specimens, having well-controlled, stable spinning rates whereby high resolution MR imaging and spectroscopy may be conducted under conditions that do not damage the tissues or cellular structure of the objects and that still further minimizes and avoid problems associated with SSBs at the required slow spinning rates, and that allows for increasing sizes of an object.